U.S. patent number 5,444,644 [Application Number 08/187,922] was granted by the patent office on 1995-08-22 for auto-configured instrumentation interface.
This patent grant is currently assigned to Johnson Service Company. Invention is credited to August A. Divjak.
United States Patent |
5,444,644 |
Divjak |
August 22, 1995 |
**Please see images for:
( Certificate of Correction ) ** |
Auto-configured instrumentation interface
Abstract
An instrumentation interface circuit controlled by a
microprocessor of a data acquisition system enables identification
of the type of input/output devices that are connected to its
inputs through a systematic analysis of the characteristics of the
connected devices, the systematic analysis including monitoring the
current through and the voltage across the devices as different
drive signals are applied to the connected devices, the interface
circuit including a conditioning circuit having a drive circuit
which is controlled by the microprocessor for providing the drive
signals, a current sensing circuit for sensing the load current
flowing through the connected device, a voltage sensing circuit for
sensing the voltage across the connected device, and a switchable
feedback circuit which is controlled by the microprocessor to
selectively feed back the load current or the sensed voltage to the
input of the drive circuit to establish the drive signals for the
connected device.
Inventors: |
Divjak; August A. (Waukesha,
WI) |
Assignee: |
Johnson Service Company
(Milwaukee, WI)
|
Family
ID: |
22691040 |
Appl.
No.: |
08/187,922 |
Filed: |
January 27, 1994 |
Current U.S.
Class: |
702/64; 324/602;
700/12; 700/32; 700/81; 700/82; 710/104 |
Current CPC
Class: |
G05B
19/0423 (20130101); G05B 2219/21083 (20130101); G05B
2219/21117 (20130101); G05B 2219/25428 (20130101); G05B
2219/2656 (20130101) |
Current International
Class: |
G05B
19/04 (20060101); G05B 19/042 (20060101); G05B
023/02 () |
Field of
Search: |
;364/152,141,483,579,580,186,187 ;324/602,130,764 ;395/325 ;340/636
;379/58 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ramirez; Ellis B.
Assistant Examiner: Peeso; Thomas
Attorney, Agent or Firm: Root, III; Joseph E. Kalinowski;
Leonard J. Levine; E. L.
Claims
I claim:
1. In a data acquisition system including a plurality of input
devices and a plurality of output devices, signal processing means
operable in a data acquisition mode for monitoring the input
devices and controlling the output devices, and a plurality of
interface circuits interposed between said input and output devices
and said signal processing means for interfacing said devices with
said signal processing means, each of said interface circuits
comprising:
conditioning circuit means having an input circuit with at least
one of said devices being connected to said input circuit,
said conditioning circuit means including drive circuit means
coupled to said input circuit for providing a drive signal for said
connected device, current monitoring means coupled to said input
circuit for providing a signal indicative of the current flowing
through said connected device, and voltage monitoring means for
providing a signal indicative of the voltage across said connected
device,
said signal processing means being programmed to be operable in a
configuring mode to control said drive circuit means to vary the
drive signal to thereby vary at least one of the current flowing
through said connected device and the voltage across said connected
device, and said signal processing means processing the signals
provided by said current monitoring means and said voltage
monitoring means as the current through and the voltage across said
connected device and are varied to determine whether said connected
device is an input or an output type device and to configure said
conditioning circuit means as a function of the type of device
connected thereto, said signal processing means identifying said
connected device as an input type device when, in the absence of a
drive signal, said voltage monitoring means provides a signal and
as an output type device when said voltage monitoring means fails
to provide a signal, and said signal processing means configuring
said conditioning circuit means to enable a connected device that
is identified as an input type device to be monitored while said
signal processing means is operating in said data acquisition mode,
and to enable a connected device that is identified as an output
type to be controlled by drive signals provided by said drive
circuit means while said signal processing means is operating in
said data acquisition mode.
2. The system according to claim 1, wherein said signal processing
means, when operating in said configuring mode, and after said
connected device has been identified as being an input type device,
is programmed to control said drive circuit means to vary the drive
signal for said connected device, by causing a signal proportional
to current flowing through said connected device to be applied to
an input of said drive circuit means while the voltage across said
connected device is monitored by said voltage monitoring means, and
by causing a signal proportional to the voltage across said
connected device to be applied to said input of said drive circuit
means while the current flowing through said connected device is
monitored by said current monitoring means, said signal processing
means comparing the signals produced by said voltage monitoring
means and said current monitoring means with reference values to
determine whether the connected device is, an externally sourced
voltage type device or externally sourced current type device,
respectively.
3. The system according to claim 2, wherein said drive circuit
means includes a differential amplifier having an inverting input,
a non-inverting input and an output, and switchable feedback
circuit means connected to said inverting input of said
differential amplifier, said non-inverting input of said
differential amplifier being connected to receive a signal provided
by said signal processing means, and circuit means connecting said
output of said differential to said input circuit; said switchable
feedback circuit means being controlled by said signal processing
means to be operable in a first mode to feed back to said inverting
input of said differential amplifier a current that is proportional
to the current flowing through said connected device and said
switchable feedback circuit means being controlled by said signal
processing means to be operable in a second mode to feed back to
said inverting input of said differential amplifier a voltage that
is proportional to the sensed voltage.
4. The system according to claim 3, wherein said circuit means
includes a low impedance circuit device connected in a series
circuit path between said output of said differential amplifier and
said connected device, said current monitoring means including an
amplifier circuit having an input and an output, said input of said
amplifier circuit being connected to said low impedance circuit
device for producing at said output of said amplifier circuit a
signal that is proportional to current flowing through said
connected device.
5. The system according to claim 4, wherein said amplifier circuit
is connected to measure the voltage across said low impedance
circuit device in producing said signal corresponding to the
current flowing through said connected device.
6. The system according to claim 4, wherein said input circuit
includes a sensing terminal, an output terminal and a reference
terminal, said connected device being connected to said output
terminal, said reference terminal being connected to a point of
reference potential, and said voltage monitoring means including
means for coupling said sensing terminal to said signal processing
means, and wherein said voltage monitoring means monitors the
potential at said sensing terminal relative to said reference
terminal.
7. The system according to claim 6, wherein the presence of a
potential difference between said output terminal and said sensing
terminal that is a fraction of the potential difference between
said first and third terminals is indicative that said connected
device is a potentiometer.
8. The system according to claim 3, wherein said signal processing
means controls said drive circuit means to establish a fixed
current through said connected device whereby the voltage across
said connected device is indicative of the resistance of said
connected device.
9. The system according to claim 3, wherein said switchable
feedback circuit means has a first input coupled to said output of
said current monitoring means, a second input coupled to said input
circuit, an output coupled to said inverting input of said
differential amplifier, and switching means controlled by said
signal processing means to connect said input circuit to said
inverting input of said differential amplifier when said feedback
circuit means in operating in said first mode, and to connect said
output of said current monitoring means to said inverting input of
said differential amplifier when said feedback circuit means is
operating in said second mode.
10. An interface circuit for coupling a plurality of input and
output devices to a signal processing means, said interface circuit
comprising:
conditioning circuit means having an input circuit adapted for
connecting at least one of said devices to said interface
circuit;
said conditioning circuit means including drive circuit means
having an output coupled to said input circuit for providing a
drive signal for said one device, current monitoring means coupled
to said input circuit for providing a signal indicative of the
current flowing through said one device, and voltage monitoring
means for providing a signal indicative of the voltage across said
one device;
said drive circuit means including a differential amplifier having
feedback circuit means including switching means, said switching
means being controlled by said signal processing means in a first
mode to feed back to said first input of said differential
amplifier a current that is proportional to the current flowing
through said one device and operable in a second mode to feed back
to said first input of said differential amplifier means a voltage
that is proportional to the across said one device, to permit said
conditioning circuit to be configured as a function of whether said
one device is an input type device or an output type device,
enabling the drive signal produced by said drive circuit means to
be selected as a function of whether said one device is an input
type device or an output type device.
11. The interface circuit according to claim 10, wherein said drive
circuit means is controllable to vary the drive signal for said one
device in a predetermined manner, whereby the values of the current
flowing through and the voltage across said one device permit
determining whether said one device is internally sourced or
externally sourced.
12. The interface circuit according to claim 11, wherein said
feedback circuit means has a first input coupled to said output of
said current monitoring means, a second input coupled to said input
circuit, an output coupled to said inverting input of said
differential amplifier, said switching means being controlled by
said signal processing means to connect said input circuit to said
inverting input of said differential amplifier when said feedback
circuit means in operating in said first mode, and to connect said
output of said current monitoring means to said inverting input of
said differential amplifier when said feedback circuit means is
operating in said second mode.
13. The interface circuit according to claim 12, wherein said drive
circuit means is adapted to establish a fixed current through said
one device whereby the voltage across said one device is indicative
of the resistance of said one device.
14. The system according to claim 10, wherein said current
monitoring means includes a low impedance circuit device and an
amplifier circuit, said circuit device being connected in a series
circuit path between said output of said differential amplifier and
said one device, and said amplifier circuit having an input and an
output, said input of said amplifier circuit being connected to
said low impedance circuit device for producing at said output of
said amplifier circuit a signal that is proportional to current
flowing through said one device.
15. The interface circuit according to claim 14, wherein said
amplifier circuit is connected to measure the voltage across said
low impedance circuit device in producing said signal corresponding
to the current flowing through said one device.
16. The interface circuit according to claim 14, wherein said input
circuit includes a sensing terminal, an output terminal and a
reference terminal, said connected device being connected to said
output terminal, said reference terminal being connected to a point
of reference potential, and said voltage monitoring means includes
means for coupling said sensing terminal to said signal processing
means, and wherein said voltage monitoring means monitors the
potential at said sensing terminal relative to said reference
terminal.
17. In a data acquisition system including a plurality of input
devices, a plurality of output devices, signal processing means
operable in a data acquisition mode for monitoring the input
devices and controlling the output devices, and interface circuit
means interposed between the input and output devices and the
signal processing means for interfacing the input and output
devices with the signal processing means, a method of determining
the types of device connected to the input of the interface circuit
means comprising the steps of:
causing said signal processing means to control a drive circuit
means of said interface circuit means to vary the current flowing
through said connected device and the voltage across said connected
device in a predetermined sequence;
monitoring at least one of the current flowing through the
connected device and the voltage across said connected device while
the current flowing through said connected device and the voltage
across said connected device are being varied;
using the current and voltage values measured to determined whether
said connected device is an input or output type device, including
identifying said connected device as an input type device when, in
the absence of a drive signal, said voltage monitoring means
provides a signal and as an output type device when said voltage
monitoring means provides a signal,
and causing said signal processing means to configure said
conditioning circuit as a function of the type of device connected
thereto, including causing said signal processing means to
configure said conditioning circuit means to enable a connected
device that is identified as an input type device to be monitored
while said signal processing means is operating in said data
acquisition mode, and to enable a connected device that is
identified as an output type device said drive circuit means to be
controlled by drive signals provided by said drive circuit means
while said signal processing means is operating in said data
acquisition mode.
18. The method according to claim 17, including causing said signal
processing means to control said drive circuit means to vary the
drive signal for said connected device in a predetermined manner,
whereby the values of current and voltage measured by said current
and voltage monitoring means are indicative of whether said
connected device is internally sourced or externally sourced.
19. The method according to claim 18, wherein controlling the drive
circuit means includes the step of feeding back a current
proportional to the current flowing through said connected device
to an input of said drive circuit means and measuring the voltage
across said connected device, and the step of feeding back a
voltage proportional to the voltage across said connected device
and measuring the current flowing through said connected
device.
20. The method according to claim 18, including the steps of
establishing a fixed current through said connected device, whereby
the voltage across the connected device is indicative of the
resistance of said connected device, and correlating the resistance
of said connected device that is indicated by said voltage with
resistance values indicative of particular types of devices for
identifying the connected device as a particular type of
device.
21. The method according to claim 18, wherein the connected device
is a relay, and including controlling the drive circuit means to
provide a drive signal that ramps up to a predetermined value for
causing the relay to operate, causing the drive signal to ramp down
for deenergizing the relay to allow the relay to release, and
observing the polarity required to release the relay, the polarity
required to release the relay indicating whether the relay is a
momentary relay or a latching relay.
Description
BACKGROUND OF THE INVENTION
This invention relates to data acquisition systems, and more
particularly to an instrumentation interface circuit for such
systems which automatically configures a plurality of input and
output devices with a microprocessor of a data acquisition system
which monitors and/or controls the input and output devices.
Environmental control systems, surveillance systems, industrial
control systems and the like, employ a plurality of different types
of transducers including input devices that are monitored by a
microprocessor to determine control functions that must be
initiated and output devices that are controlled by the
microprocessor to implement the control functions that are
required. These input and output devices can be classified into
four general categories, namely internally sourced input devices,
internally sourced output devices, externally sourced input devices
and externally sourced output devices. Internally sourced input
devices include RTD temperature sensors, pressure transducers,
potentiometers and dry contact inputs. Internally sourced output
devices include transducers requiring a 4 to 20 milliamp output or
1 to 10 volt output and include momentary relays and latching
relays. Externally sourced input devices are configured to provide
4 to 20 milliamp current inputs and 1 to 10 volt inputs to the
interface circuit. Externally sourced output devices are configured
to modulate a power source to conduct 4 to 20 milliamps.
Because each of these input and output devices has different
characteristics, an instrumentation interface must be provided
between the input and output devices and the microprocessor which
monitors and controls the input and output devices. Typically, such
instrumentation interface includes a plurality of analog to digital
converters, a plurality of digital to analog signal
converter/driver circuits and a plurality of conditioning circuits,
including a conditioning circuit individually associated with each
input/output device and adapted to the characteristics of the
associated input/output device. The input and output devices are
connected to the associated conditioning circuit which is
interposed between the device and an analog to digital converter
when the connected device is an input/type device and which is
interposed between the device and a digital to analog signal
converter/driver circuit when the connected device is an output
type device. The analog to digital converter with its associated
conditioning circuit converts input signals, or indications such as
contact closures, into a digital sense signals for use by the
microprocessor. The digital to analog signal converter/driver
circuit responds to digital control signals provided by the
microprocessor to provide suitable drive signals via the associated
conditioning circuit for the output device connected to the
conditioning circuit.
Typically, the data acquisition system includes a multiplexing
arrangement which permits several of the same type of input devices
to use a common analog to digital converter. However, a separate
conditioning circuit has to be provided for each device because the
conditioning circuit must be specific to the type of input/output
device that is connected to the conditioning circuit. For example,
the conditioning circuits that are connected to current input
devices, must include current detecting means, and the conditioning
circuits that are connected to voltage input devices must include a
voltage detecting means. Similarly, each output device requires a
separate digital to analog/driver circuit because the digital to
analog driver circuit must be matched to the device that it drives.
Accordingly, because of the large number of input/output devices
employed in a data acquisition system, many different types of
conditioning circuits and driver circuits must be provided to
properly interface the different types of input/output devices with
the microprocessor. Consequently, considerable planning is required
in the layout and installation of the data acquisition system
because of the need to associate the large number of input and
output devices with their conditioning circuits in making the
connections of each input/output device to its designated
conditioning circuit.
SUMMARY OF THE INVENTION
The present invention provides an auto-configured instrumentation
interface circuit for a data acquisition system which monitors
and/or controls a plurality of input/output devices, which may
include internally sourced output devices, internally sourced input
devices, externally sourced output devices and externally sourced
input devices. During installation and/or initialization of the
data acquisition system, the instrumentation interface circuit is
controlled by a microprocessor of the data acquisition system to
automatically determine the type of input/output device that is
connected to its inputs through a systematic analysis of the
characteristics of the connected device or devices, and provides
the proper input or output to each of the connected devices.
The instrumentation interface circuit includes an analog to digital
converter, a digital to analog converter and conditioning circuit
means which are controlled by a microprocessor of the data
acquisition system. The conditioning circuit means includes input
means, current monitoring means and voltage monitoring means. The
input means provides terminations for at least one connected
input/output device. The current monitoring means and the voltage
monitoring means provide inputs to the analog to digital converter
corresponding, respectively, to the current flowing through and the
voltage across the connected device.
In carrying out the systematic analysis, the microprocessor causes
each connected input device to be energized in a predetermined
sequence while monitoring the output of the voltage monitoring
means and of the current monitoring means to identify the device as
a voltage input or current input type device, a voltage output or
current output type device or as a resistance type device. During
the systematic analysis, a voltage that is proportional to the
sensed voltage is fed back to the drive circuit while monitoring
the current flowing through and the voltage across the connected
device, and a current that is proportional to the current flowing
through a load is fed back to the drive circuit while monitoring
the current through and the voltage across the connected device.
The systematic analysis is carried out in a preestablished sequence
of steps, and involves selecting whether voltage or current is fed
back while controlling the digital to analog converter to cause the
signal output provided by the driver circuit to produce a
measurable output condition for the connected device that is
indicative of the type of device that is connected to the
conditioning circuit. In accordance with a disclosed embodiment,
first a determination is made as to whether or not the connected
device is an output device. This determination includes
distinguishing between voltage and current output type devices.
Then, a determination is made as to whether the device is an input
device, including distinguishing between voltage input, current
input and resistance type devices. The resistance determination
includes identifying the specific type of resistance device, such
as an RTD sensor, a potentiometer, or an open contact or a closed
contact, by discriminating between ranges of resistance values
measured.
The invention consists of certain novel features and structural
details hereinafter fully described, illustrated in the
accompanying drawings, and particularly pointed out in the appended
claims, it being understood that various changes in the details may
be made without departing from the spirit, or sacrificing any of
the advantages of the present invention.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an instrumentation interface circuit
provided by the present invention;
FIG. 2 is a simplified schematic circuit diagram of the
conditioning circuit of the instrumentation interface circuit shown
in FIG. 1;
FIG. 3 is a detailed schematic circuit diagram of the conditioning
circuit;
FIGS. 4 and 4A are a process flow chart illustrating the process of
identifying universal input/output devices that are connected to
the interface circuit provided by the present invention; and,
FIG. 5 is a waveform of the drive signal for a relay that is
connected to the interface circuit as an output device,
illustrating the pick-up voltage and the drop out voltage for the
relay.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to the FIG. 1 of the drawings, the instrumentation
interface circuit 10 provided by the present invention includes
conditioning circuits 12-1 to 12-4, an analog to digital converter
14, digital to analog converters 16-1 to 16-4, and multiplexing
circuit 18. Typically, the components of the interface circuit 10
are mounted on a printed circuit board (not shown). The components
of the interface circuit 10 are controlled by a microprocessor or
signal processing circuit 20 of a data acquisition system during
installation and/or initialization of the system to identify one or
more universal input/output devices that are connected to the
interface circuit. By way of example, the interface circuit 10 may
include four conditioning circuits 12-1 to 12-4, one analog to
digital converter 14, four digital to analog converter circuits
16-1 to 16-4, and an eight channel multiplexing circuit 18, with
the conditioning circuits 12-1 to 12-4 being adapted to provide
connections to any combination of four input devices or output
devices 19-1 to 19-4. By universal input/output device is meant
that the four connected devices for any one of the interface
circuits can be voltage input devices, current input devices,
voltage output devices, current output devices, or a mixture of
these types of devices.
Each of the conditioning circuits, such as conditioning circuit
12-1, has a sense input terminal 21, an output or source terminal
22, and a common terminal 23 which provide connections for its
associated field device or input/output device 19-1. The field
connections are terminated by at least three wire connections to
cover most input type devices and at least two terminations for
output type devices. The conditioning circuit 12-1 has three output
terminals 24, 25 and 26 which are connected to respective inputs
27, 28 and 29 of the multiplexing circuit 18. The multiplexing
circuit 18 has an output 30 connected to an input 31 of the analog
to digital converter 14 and two channel select inputs 34 and 35
which are connected to output ports 36 and 37 of the microprocessor
20. The microprocessor has further output ports 38 and 39 which are
connected to a gain control input 42 and a feedback select input
41, respectively, of the conditioning circuit 12-1. The processing
circuit is connected to a data output 47 of the analog to digital
converter 14 and to a data input 48 of the digital to analog
converter 16-1 via data bus 45. The digital to analog converter has
an output 49 which is connected to an input 51 of the conditioning
circuit 12-1.
The interface circuit 10, under the control of the microprocessor,
determines the type of devices 19-1 to 19-4 that are connected to
its input terminals 21-23 and provides the proper input or output
to the connected devices 19. When a connected device is identified
as being an input type device, the interface circuit acts as a
voltage sensing circuit or a current sensing circuit depending upon
whether the connected device is identified as a voltage or current
type device. Similarly, the interface circuit provides a voltage or
current drive signal to the connected device depending upon whether
the connected device is identified as a voltage or current type
device. The interface circuit determines the type of device that is
connected to its input terminals by analyzing the characteristics
of the device using a systematic analysis of the voltage across and
the current through the device.
The interface circuit identifies four types or categories of
devices involved, namely, internally sourced input devices which
provide an input to the interface circuit, internally sourced
output devices which receive a drive output from the interface,
externally sourced input providing devices and externally sourced
output devices which receive an input from the interface circuit.
Internally sourced input providing devices include RTD temperature
sensors, pressure transducers, potentiometers and dry contact
inputs. The internally sourced output devices include transducers
requiring a 4 to 20 milliamp output or 1 to 10 volt output and
include momentary relays and latching relays. Externally sourced
input providing devices are configured to provide 4 to 20 milliamps
current inputs and 1 to 10 volt inputs to the interface circuit.
Externally sourced output devices are configured to modulate a
power source to conduct 4 to 20 milliamps. Such systems employ a
plurality of different types of transducers including input
devices, such as RTD temperature sensors, humidity sensors,
pressure transducers, smoke stack temperature transmitters,
potentiometers, relays providing indications via contact closures,
and other devices for providing an outputs that are indicative of
conditions being monitored. The systems also include a plurality of
output devices, such as fan speed controllers, proportional
controllers, relays, solenoid operated valves, etc., which are
controlled to carry out desired functions.
Referring to FIG. 2, which is a simplified schematic circuit
diagram of the conditioning circuit 12-1 of the instrumentation
interface circuit, the conditioning circuit 12-1 comprises a drive
circuit 60 including a differential amplifier 61, an
instrumentation amplifier 62, a resistor 64, and a switchable
feedback circuit 66. The switchable feedback circuit 66 is
represented by a double pole single throw switch 68 which is
operated by a microprocessor generated control signal supplied to
the switch 68 on input 42.
The non-inverting input 72 of the differential amplifier 61 is
connected to terminal 51 which is connected to the output 49 of the
digital to analog converter 16-1. The output 74 of the drive
circuit 60 is connected through resistor 64 to terminal 22. In FIG.
2, terminal 22 is shown connected to the sense terminal 21 by a
link or jumper 71. The link 71 is installed by the field personnel,
depending on the type of input/output device that is connected to
the conditioning circuit 12-1.
The resistor 64 is connected across the inputs 62a and 62b of the
instrumentation amplifier 62. The resistor 64 has a low resistance
value, 15 ohms, in the exemplary embodiment. The resistance value
of resistor 64 is selected to be low enough to enable a desired
current level to be established through the circuit from the output
of the drive circuit 60 to the input terminal 22 and through the
connected device, but to be high enough to protect circuit devices
from excess current.
The output 63 of the instrumentation amplifier 62 is connected to
terminal 24 which is connected to input 27 of the multiplexing
circuit 18 to provide a current input to the analog to digital
converter. The sense input terminal 21 is connected to at terminal
26 to input 29 of the multiplexing circuit 18 to provide a voltage
input to the analog to digital converter that corresponds to the
sensed voltage V.sub.s that is provided at terminal 21.
Referring to the switchable feedback circuit 66, the switch 68 has
a switch arm 74, a pole 80 connected to the output 63 of the
instrumentation amplifier 62 and a pole 82 connected to the
terminal 22 via conductor 86. The switch arm is connected to ground
through series connected resistors 75 and 76. The inverting input
78 of the differential amplifier of the drive circuit 60 is
connected to the junction of resistors 75 and 76. The value of
resistor 75 is twice the value of resistor 76 so that under voltage
feedback conditions, the voltage supplied to the inverting input of
the differential amplifier 61 of drive circuit 60 is one-third the
value of the sensed voltage. The relative values of the resistors
75 and 76 provide a gain of three for the differential
amplifier.
The switchable feedback circuit 66 provides feedback of the voltage
V.sub.s sensed at terminal 22 to the inverting input of the
differential amplifier of the drive circuit 60 when the switch arm
74 is operated to engage pole 82. The switchable feedback circuit
66 provides feedback of current proportional to the load current
I.sub.L from the output of the instrumentation amplifier 62 to the
inverting input of the differential amplifier 61 when the switch
arm 74 is operated to the position illustrated in FIG. 2 to engage
pole 80. The current fed back corresponds to the load current,
i.e., the current flowing through resistor 64 from the differential
amplifier 61, which has a gain of three, as amplified by the
instrumentation amplifier. When current feedback is being provided
to the differential amplifier 61, the effective value of resistor
64 as seen by the differential amplifier is five ohms.
The microprocessor 20 is programmed to analyze the devices 19-1 to
19-4 which are connected to the inputs of the interface circuit 10
to identify the device type by determining the device is sourced
internally or externally and by monitoring the response of the
connected device under current and voltage feedback conditions for
each connected device. The microprocessor goes through an
evaluation to test first if a connection has been made to the
conditioning circuit input or output terminals, and if so to then
determine the type of device that is connected to the conditioning
circuit. The interface circuit 10 measures the voltage V.sub.s at
terminal 22 relative to ground or the common reference and the
current flowing through the resistor 64, which corresponds to the
current that is being supplied to the device 19-1 connected between
terminal 22 and the ground terminal 23. The voltage and current
values are read by the microprocessor 20 via the analog to digital
converter circuit. During the analyzing sequence, the
microprocessor 20 controls the switchable feedback circuit 66 to
select voltage and/or current feed back to the drive circuit 60.
The microprocessor 20 also controls the gain of the instrumentation
amplifier circuit to control resolution by providing a large
dynamic range for measurement of the sensed voltage and/or load
current. The drive circuit 60 is embodied as a differential
amplifier, but other types of summing or combining circuits may be
used to combine the control signal provided by the microprocessor
with the feedback signal provided by the feedback circuit 66.
Referring to FIG. 3, there is a detailed schematic circuit diagram
of one an implementation of the conditioning circuit 12 shown in
FIG. 2. The conditioning circuit shown in FIG. 3 has been given the
reference numeral 85 and components of the conditioning circuit 85
have been given the same reference numerals as corresponding
components of the conditioning circuit 12 shown in FIG. 2. The
conditioning circuit 85 includes a power amplifier stage 86 which
is interposed between the output of the differential amplifier and
the load resistance 64. The instrumentation amplifier 62 comprises
three operational amplifiers 88, 89 and 90. Also, the feedback
selector switch 68 is implemented as a pair of latch circuits 91
and 92, latch circuit 91 being interposed between the output of the
instrumentation amplifier and the inverting input of the
differential amplifier 61 and the latch circuit 92 being interposed
between the output of amplifier 89, which provides an output that
is proportional to the sensed voltage V.sub.s and the inverting
input of the differential amplifier 61. Also, the instrumentation
amplifier includes a latch circuit 93 that is associated with the
output stage operational amplifier 90 and is enabled by a control
signal applied to gain control input 42 by the microprocessor to
connect a resistor 94 in circuit with the gain control feedback
loop of the amplifier 90, to change the gain of the instrumentation
amplifier when greater resolution is required. The operation of the
conditioning circuit is readily understood from the foregoing
description of the conditioning circuit 12-1, and accordingly will
not be described herein.
TABLE 1 illustrates the types of input and output configurations
which can be accommodated by the interface circuit by selecting
voltage and current feedback along with values for the signal
output of the digital to analog converter 16 which is supplied to
the drive circuit 60.
For purposes of illustration of the operation of the interface
circuit, it is assumed that the microprocessor operates in the
manner set forth in the process flow chart given in FIGS. 4 and 4A.
It is assumed that terminals 21 and 22 are interconnected by link
71. The microprocessor is programmed to sequentially test for the
presence of a voltage input device or voltage output device, a
current input device or a current output device, and a resistance
device, such as an RTD temperature sensor, a potentiometer or a
contact input. The microprocessor 20 is programmed to determine the
ratings and type, i.e., a momentary or a latching relay, for a
relay having its operate winding connected across the terminals 22
and 23. The disclosed sequence of testing operations is provided by
way of example, and other test operation sequences may be used, for
example, testing first for resistance type devices, or testing
first for current input or output type devices, etc.
Referring to FIG. 1, before starting the test sequence to identify
the device connected to each set of input terminals of the
interface circuit 10, the microprocessor determines whether or not
devices are connected to the sets of input terminals of the
interface circuit 10, as well as to the sets of input terminals of
all other interface circuits of the data acquisition system. During
installation and/or initiation of the data acquisition system, all
of the connected devices are routinely activated by personnel
installing the system, creating detectable changes in the status or
condition of the point. The microprocessor routinely looks at all
points and stores an indication in a memory 73 of the
microprocessor if there is a change in the status or condition of a
point, i.e., a change in the potential of a source terminal 22
relative to ground terminal 23, a change from an open circuit
condition to a closed circuit condition, etc., the latter change
indicating that the point, i.e. the connected device, is a relay
contact. If a device other than a normally open relay contact is
connected to the set of inputs, the device can be identified by the
interface circuit 10, under the control of the microprocessor, by
the systematic analysis of its response to impressed voltage and/or
current conditions as will be shown. If there is no change in the
status of the point indicative that the point is always "open",
this is an indication that there is no device connected to that
particular set of input terminals.
Referring additionally to FIG. 2, and to the process flow chart
given in FIGS. 4 and 4A, in the exemplary test sequence, at block
81, the microprocessor initially configures the conditioning
circuit 12-1 to test for an externally sourced input type device.
The microprocessor provides a control signal on channel select
input 35 to connect the voltage sensed input at terminal 26 to the
input of the analog to digital converter 14. At decision block 97,
the microprocessor monitors the output of the analog to digital
converter to test for the presence of a voltage at the sense
terminal 21. If a voltage is present on the voltage sense input,
the device is an externally sourced input device and a
determination is made as to whether the device is a voltage or
current input type device.
At block 98, the microprocessor provides a control signal on the
feedback select input 41 of the switch 68 to cause the switch to
connect output 63 of the instrumentation amplifier to the inverting
input 78 of the differential amplifier 61, providing current
feedback to the input of the drive circuit 60. Also, the
microprocessor provides a control signal on select input 35 of the
multiplexing circuit 18 to connect terminal 26, and the sense
terminal 21, to the input of the analog to digital converter 14,
for monitoring the sensed voltage V.sub.s provided across the
terminal 21 relative to ground. The microprocessor provides an
output via data bus 45 to the input of the digital to analog
converter 16 (FIG. 1 ), setting the output of the digital to analog
circuit to zero causing the output of the differential amplifier to
zero, resulting in no current flowing through the load resistor 64
to terminal 21. Thus, the differential amplifier output provides a
high output impedance to the load, including resistance 64 and the
connected device 19. At decision block 99, the microprocessor reads
the output of the analog to digital converter and if the output is
a voltage in the range of 1 to 10 volts, the connected device 19
has been identified as being a voltage input type device, block
100. Otherwise, a test is made to determine if the device is an
externally sourced current input device.
At block 101, the microprocessor sets the switch 68 to provide
voltage feedback to the inverting input of the differential
amplifier 61 and provides a control signal to channel select input
34 of the multiplexing circuit 18 to cause the output of the
instrumentation amplifier to be connected to the input of the
analog to digital converter. Also, the microprocessor sets the
output of the digital to analog converter to zero, causing the
differential amplifier to provide a zero current output. With
voltage feedback to the differential amplifier, a virtual ground is
provided at the output of the drive circuit 60. If the connected
device is an externally sourced current input device, current will
flow through the load resistor 64 from terminal 22 to the output of
the drive circuit. This current is detected by the instrumentation
amplifier 62 so that current is provided to terminal 24. The output
of the analog to digital is monitored by the microprocessor and if
the current is in the range of 4-20 milliamps, block 102, the
connected device has been identified as being an externally sourced
current input device, block 103.
Referring back to decision block 97, if no voltage is sensed at the
sense terminal 21, the connected device is not an externally
sourced input type device and so at block 104 the microprocessor
configures the conditioning circuit to test for a voltage or
current output device or a resistance type device. For this portion
of the test sequence, the microprocessor sets the feedback select
switch 68 to provide voltage feedback to the differential amplifier
and provides a control signal on the channel select input 35 of the
multiplexing circuit 18 to connect terminal 26 to the input of the
analog to digital converter 14, for monitoring the sensed voltage
V.sub.s. The microprocessor sets the digital to analog converter to
provide a voltage corresponding to the feedback voltage, i.e.,
one-third the sensed voltage, so that the drive circuit 60 supplies
a current of approximately 1 milliamp through the load resistor 64.
The current flowing through the load resistor, and thus through the
connected device 19, is monitored by the instrumentation amplifier
62. If the sensed voltage V.sub.s is greater than 10 volts, then at
decision block 105, the connected device has been identified as
being a voltage output type device, block 106. If not, a test
sequence is carried to attempt to identify the device as a current
output type device.
If the voltage is not greater than 10 volts, a test is made to
determine if the connected device is a current output device. At
block 107, the microprocessor sets switch 68 to provide current
feedback to the drive circuit 60 and controls the digital to analog
converter to provide an output current that is proportional to the
load current I.sub.L, and a factor which takes into account the
gain of the differential amplifier 61. This current is represented
by the relationship I.sub.L * A.sub.v * 5, where A.sub.v is the
gain of the differential amplifier 61, which is three in the
exemplary embodiment and the constant "5" is selected to make the
load resistance 64 appear to have an effective value of 5 ohms. The
current causes the driver circuit to provide a 4 milliamp current
through the load resistor 64 to sense terminal 21 which is verified
by monitoring the output of the analog to digital converter to
confirm that the load current is 4 milliamps. Then the sensed
voltage V.sub.s is monitored. If, at decision block 108, the
voltage sensed is that provided for a load impedance on the order
of 500 to 600 ohms when a 4 milliamp current is flowing in the load
circuit, the connected device has been identified as being a
current output device, block 109. If not, the connected device is a
resistance type device or relay contact, and tests are carried out
to determine the type of resistance device.
At block 110 the microprocessor sets switch 68 to provide current
feedback and sets the digital to analog converter to cause a 1
milliamp current to flow through the load resistor 64 to terminal
22. Then, the voltage output of the analog to digital converter is
compared with a reference value, at decision block 111, to
determine if the impedance of the device is in the range of
approximately 680 ohms to 1500 ohms. If so, the device is a RTD
temperature sensor as indicated at block 112. If not, a comparison
is made at block 113 to determine if the impedance is less than
about 10 ohms or greater than approximately 100K ohms. If so, the
device 19 is a contact input as indicated at block 114.
If the voltage V.sub.s is a percent of the input voltage and the
impedance is in the range of approximately 100 to 5000 ohms, the
connected device 19 is identified as being a potentiometer. When
the connected device is a potentiometer, the tap of the
potentiometer is connected to the sense terminal 21 and the
resistance portion of the potentiometer is connected between
terminals 22 and 23. Thus, the link 71 is not provided between the
source terminal 22 to the sense terminal 21. Therefore, if at
decision block 115 the sensed voltage V.sub.s is determined to be a
percent of the input voltage, the connected device is identified as
being a potentiometer as indicated at block 116.
If the impedance is in the range of approximately 25 to 500 ohms
and is slightly inductive, the connected device 19 is identified as
being a relay, block 117. If the connected device is the operate
winding of a relay, a detectable spike 118 will be produced in the
sensed output voltage when the relay operates, as is shown in FIG.
5. If the device is identified as being a relay, a test is made to
determine if the relay is a momentary relay or a latching relay. To
test for the relay type, the microprocessor controls the digital to
analog convertor to energize the drive circuit 60 to provide a
drive current (or voltage) that ramps up to a maximum value while
monitoring the load voltage (or current). If the connected device
is identified as the operate winding of a relay, a determination is
made as to whether the relay is a momentary relay or a latching
relay. To this end, the microprocessor controls the digital to
analog converter 18 to cause the drive circuit 60 to provide a
drive signal that ramps up to energize the relay and ramps down to
deenergize the relay, at block 119. When the relay operates, the
spike 118 is produced in the output voltage indicating the turn on
voltage for the relay. When the drive signal is ramped down, a
further spike 118a is produced when the relay releases. For a
momentary relay, the spike 118a is produced as soon as the drive
voltage begins to decrease. If this is detected at decision block
120, it has been determined that the device is a momentary relay as
indicated at block 121. However, for a latching relay, no spike is
produced until the current polarity is reversed, producing the
second spike 118a. If such a spike is detected when ramping down
the current, it has been determined that the device is a momentary
relay as indicated at block 122. If no spike is detected until the
current is ramped in the opposite direction, it is determined that
the device is a latching relay. Thus, a determination of whether
the relay is a momentary relay or a latching relay can be made by
observing the polarity required to release the relay because the
polarity required to release the relay indicates whether the relay
is a momentary relay or a latching relay.
If the device fails to meet any of the test criteria, it is assumed
that the device is not a valid input/output type device and the
select routine is ended and an indication is provided that no valid
input/output device is connected to the conditioning circuit being
analyzed. In all of the test sequences, the ranges of voltage,
current and resistance can be tested using case statements of the
program or in any other suitable manner such as using table
look-ups, etc. One example of an impedance determination subroutine
is given by the source code provided in Appendix 1.
After the type of each input/output device connected to the
interface circuit has determined through the systematic analysis of
the characteristics of the connected device or devices, information
relating to the identification and operating characteristics of the
devices is stored in memory by the microprocessor to enable the
microprocessor to provide the proper input or output to each of the
connected devices. That is, input type devices are configured to be
connected to the analog to digital converter, output type devices
are configured to receive an output via a digital to analog
converter, etc. After the types of devices have been established,
the voltage and/or currents are continually monitored by the
microprocessor for diagnostic evaluation as well as reporting on
the input and/output values. The configuration information derived
is stored in memory and the point interface continues in the
configured mode until an error is detected. This error would
indicate some type of fault condition or a change in a connected
device. The system would then go through another configuration
operation sequence, performing a systematic analysis of the
characteristics of the connected device or devices to determine the
new point type or the type of fault encountered. Such systematic
analysis can be carried out periodically or only in response to the
detection of an error condition.
APPENDIX I
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DETZ: SELECT CASE FZ CASE IS > 50000! IF AST 7 THEN ERRFG = 1
ACT = 7 OTYPE = 4 FVAL = .005 DVAL$ = "OPEN" DTYPE$ = "SWITCH" CASE
3000 TO 50000 IF AST 2 THEN ERRFG = 1 ACT = 2 OTYPE = 2 FVAL = 1
FIO = FI DTYPE$ = "VOLTAGE OUTPUT 1-10V Range" DVAL$ =
STR$(CLNG(FVAL * 1000)/1000) + "Volts" CASE 680 TO 1510 IF AST 8
THEN ERRFG = 1 ACT = 8 OTYPE = 4 FVAL = .0001 DVAL$ = STR$(FNFTEMP)
+ CHR$(248) + "F" DTYPE$ = "RTD TEMPERATURE ELEMENT" CASE 495 TO
680 ACT = 4 OTYPE = 4 DVAL$ = STR$(FVAL * 1000) + "ma." DTYPE$ = "4
TO 20 ma. LOOP (driven)" CASE 405 TO 495 FRON = -12 FRION = FRON/FZ
FRSS = .5 FRISS = FRSS/FZ OTYPE = 6 ACT = 6 CALL TSTLATCH(ACT,
FRON, FROFF, FRIOFF, RTYPE$) FVAL = FROFF OTYPE = 6 DVAL$ = "OFF"
DTYPE$ = "12V RELAY" + RTYPE$ CASE 180 TO 250 FRON = 12 FRION =
FRON/FZ FRSS = .5 FRISS = FRSS/FZ OTYPE = 6 ACT = 5 CALL
TSTLATCH(ACT, FRON, FROFF, FRIOFF, RTYPE$) FVAL = FROFF DVAL$ =
"OFF" DTYPE$ = "12V RELAY" + RTYPE$ CASE 108 TO 180 FRON = 5 FRION
= FRON/FZ FRSS = .5 FRISS = FRSS/FZ OTYPE = 6 ACT = 6 CALL
TSTLATCH(ACT, FRON, FROFF, FRIOFF, RTYPE$) FVAL = FROFF DVAL$ =
"OFF" DTYPE$ = "5V RELAY" + RTYPE$ CASE 73 TO 89 FRON = -6 FRION =
FRON/FZ FRSS = .5 FRISS = FRSS/FZ OTYPE = 2 ACT = 6 CALL
TSTLATCH(ACT, FRON, FROFF, FRIOFF, RTYPE$) FVAL = FROFF DVAL$ =
"OFF" DTYPE$= "6V RELAY" + RTYPE$ CASE 25 TO 35 FRON = 6 FRION =
FRON/FZ FRSS = .5 FRISS = FRSS/FZ ACT = 5 OTYPE = 6 CALL
TSTLATCH(ACT, FRON, FROFF, FRIOFF, RTYPE$) FVAL = FROFF DVAL$ =
"OFF" DTYPE$ = "6V RELAY" + RTYPE$ CASE 0 TO 10 IF AST 7 THEN ERRFG
= 1 ACT = 7 DVAL$ = "CLOSED" DTYPE$ = "SWITCH" CASE ELSE ACT = 0
DVAL$ = STR$(FZ) + "OHMS" DTYPE$ = "NOT A VALID I/O TYPE" END
SELECT IF ERRFG = 1 THEN COMNT$ = "LOAD INCONSISTANT-TYPE ASSUMED"
CALL DISPLAY(COMNT$, DVAL$, DTYPE$) GOTO OPERATE
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